1. Field of the Invention
This invention relates to a process for fusion-welding a binary combination of an iron alloy containing 50% by weight or less of nickel and cobalt combined and copper, or a ternary combination of said iron alloy, copper, and iron by bringing members of said binary or ternary combination into gapless contact with one another and subjecting the contact zone to a treatment selected from electron beam welding, laser beam welding, plasma welding, and TIG welding without use of a filler metal.
2. Description of the Prior Art
In an electromagnetic wave generating device (magnetron) and a semiconductor device for use in an electronic range, it becomes necessary to make a joint by brazing between a ceramic insulator and a Co-Ni-Fe alloy such as Fernico or Kovar. Because of its thermal expansion coefficient approximating to that of ceramics, the Co-Ni-Fe alloy is widely used in the above-noted field.
In the semiconductor device and magnetron, welding of the Co-Ni-Fe alloy with copper or with both copper and iron is also conducted. In welding together a Co-Ni-Fe alloy and copper or Co-Ni-Fe alloy, copper, and iron, it is concievable to conduct the welding by forming weld beveling and by use of a filler metal. However, in such a case as in electronic parts where the parts to be welded are small in size and complicated in structure, it is inadequate to form a weld beveling. In an electronic device, deformation and strain build-up in its parts caused by the weld beveling should be avoided. It is desirable, therefore, if it is possible to make a butt welding or lap welding or to effect edge welding, without forming a weld beveling. The welding techniques adoptable for such purposes are electron beam welding, laser beam welding, plasma welding, and TIG welding without filler metal.
When an assembly of two base metals of copper and an Fe-Ni-Co alloy or of three base metals of copper, iron, and an Fe-Ni-Co alloy is fusion-welded by any of the methods comprising electron beam welding, laser beam welding, plasma welding, and TIG welding without using a filler metal, generally there occurs cracking in the weld metal. When observed under a microscope, the portion where cracking has occurred reveals a structure in which crystal grains of a metal containing mainly iron are surrounded by a solid solution in the form of film of a metal containing copper as the main constituent, and cracks are formed along the boundaries of crystal grains. From the observation, it is presumable that formation of cracks are due to the fact that in the course of solidification of the molten metal, a metal containing mainly iron, which has crystallized in the liquid phase begins to contract and so the remaining liquid phase is subject to an increasing tensile stress and at the final stage of solidification said stress gradually increases and when the liquid phase at grain boundary cannot stand the contraction, crevices occur.
In order to avoid cracking in a weldment, it is generally effective to add to the weld metal from an external source metal capable of preventing development of cracks. Such a means, however, lacks in reliability because of inclusion of a metal having a third composition different from the base metals, which probably tends to change mechanical or physical properties of the weldment, and, moreover, it is technically difficult to select the type of metal to be added and the means of adding such a metal. Under the circumstances, it is most desirable for the countermeasure against cracking to find a weld metal composition which resists cracking without addition of a metal from external sources and to find a welding method which is able to result in said composition in the weld metal.